The ongoing focus on miniaturisation and the increasing complexity of integrated circuits demands for a continuous higher density integration. To achieve this, there is an ongoing downscaling in the dimensions of the active devices as well as of the structures interconnecting these devices. These interconnect structures can comprise multiple metal layers which are, dependent on the desired interconnect pattern, either separated one from another by means of interlayer insulating layers or connected one to the other by means of a connection through the insulating layer. To provide such a connection, first openings are formed in the insulating layer and filled thereafter with a conductive material. Examples of such openings are via holes or contact holes or trenches. To meet the high density integration requirements, the diameter of these openings is decreasing, while at the same time the aspect ratio of these openings is increasing. Furthermore also the number of openings per unit area is increasing due to the high density requirements and therefore also due to the growing number of metal layers. As a consequence the filling process of these openings and in particular the filling yield, reliability and speed are becoming more and more critical.
Classically, the filling process of these openings is a process wherein an Al-containing metal is used to fill these openings. Several methods, i.e. filling processes, have been introduced which use conventional Physical Vapour Deposition, i.e. no directional sputtering by means of e.g. a collimator. Ono et al., Proc. VMIC, 1990, p. 76, describe a method wherein the Al is deposited onto the wafer at a low deposition rate in a single-step process. The wafer is kept at a high temperature of about 500.degree. C. during deposition to ensure contact hole/via fill (Ono et al., Proc. VMIC, 1990, p. 76). Although it is possible to fill the openings with this one step process, one of the inherent disadvantages is that this process is too slow. Another disadvantage is that openings with different aspect ratios cannot be efficiently filled simultaneously on a wafer as required in a modern metallization scheme such as dual damascene.
In another method, as in Park et al., Proc. VMIC, 1991, p. 326, the Al film is first sputter deposited cold. In order to fill the contact openings and/or via openings, the wafer is heated in situ after this deposition in order to reflow the Al into the contact openings and/or via openings. In yet another method, as described in e.g. the United States patent applications U.S. Pat. Nos. 5,371,042 and 5,270,255, a thin and continuous Al-containing seed layer is sputtered cold onto a wetting layer formed in these openings. This sputtering is performed cold in order to avoid dewetting of the Al and the TiAl.sub.3 formation. This wetting layer typically consists of either a single Ti layer or a triple barrier layer with a Ti rich surface. After deposition of the seed layer, the remainder of the Al-containing layer is sputtered at an elevated temperature, i.e. usually in the range of 400-500 .degree. C., and slowly in order to give the material enough time to flow into the opening.
These methods run into problems when the diameter of the opening decreases, particularly when the diameter is in the sub 0.5 .mu.m range and/or for aspect ratios of 2:1 or higher. Because of the increased aspect ratio the deposition of a continuous Al-containing seed layer on the inner walls of the openings by line of sight sputtering is hampered. In fact, during the seed layer formation, the formation of an additional overhang on the top of the small features is initiated. What is typically observed in case of this conventional sputtering into an high aspect ratio opening is that bridging of the top of the via occurs and that a void is created. The void will then be filled by way of Al-containing bulk diffusion, i.e. from top to bottom, which is inherently slower than Al-containing surface diffusion, i.e. from bottom to top, in the temperature range used in the filling process. Consequently yield and reliability of this filling process for openings with high aspect ratios and sub 0.5 .mu.m diameters is questionable, whereas the productivity is far too low.